Transformer



May 25, 1948.

M. B. MALLETT TRANSFORMER 8 Sheets-Sheet 1 Filed June 16, 1944 .Yfzzzaziuz" MWMLLEEMLETT M. B. MALLETT TRANSFORMER May 25, 1948.

8 Sheets-Sheet 2 Filed June 16, 1944 May 25, 1948. M. B. MALLETT TRANSFORMER Filed June 16, 1944 8 Sheets-Sheet 3 HGH VOLTAGE HIGH VOLTAGE LINE NEUTRAL JHUEHE'UP Hamil/[LLB EMELLETT May 25,1948. 7' M, MALLETT 2,442,274

TRANSFORMER Filed June 16, 1944 8 Sheets-Sheet 4 25 I i 24 I8 CONVENTIONAL TYPE. LOW VOLTAGE W\ND\N6 5225172 01" MUDJZFILLEEMELLETT May 25, 1948. M. B. MALLETT TRANSFORMER Filed June 16, 1944 8 Sheets-Sheet 5 LO W V0 LTA GE liwan'zur Nani/ILL: EMELLBTT May 25, 1948. M. B. MALLETT TRANSFORMER Filed June 16, 1944 8 Sheets-Sheet 6 HIGH VOLTAGE LINE Fm 1mm H WM Em. WV/V/V/I/VVVVVV/ E i uw .ro o B m E m. L m D L E Iiilil M m n O V H 1 L m T m N L E m m w w w E H N m E H, m M m h H H m u E v. M G H Y L A m0 50 23 m l m 33% w m H ONE LEG OF THREE PHASE DESIGNS (OR ONE OF TWO MULTIRLE'LEGS SINGLE PHASEDEEIGN) May 25, 1948. MALLETT 2,442,274

TRANSFORMER Filed June 16, 1944 8 Sheets-Sheet 7 May 25, 1948. M. B. MALLETT TRANSFORMER Filed'June 16, 1944 8 Sheets-Sheet 8 Jill/5.172 01" MDHE'UILLE EH51, LETT Patented May 25, 1948 TRANSFORMER Montville Burgess tario, Canada, Company assig of Canada Mallett, St. Catharines, On nor to English Electric Limited, St. Catharines,

Ontario, Canada, a corporation of Canada Application June 16, 1944, Serial No. 540,631

2 Claims.

This invention relates to improvements in transformers, particularly to power transformers of the core type.

The objects of the invention are to provide a transformer of increased efficiency and simplified construction which may be manufactured economically.

Power transformers for service in the higher voltage transmission systems generally operate with grounded neutral star connected high voltage windings, and the core type transformer design embodying the present invention is particularly useful with such a connection.

In most core type power transformers, as usually manufactured on this continent, the high voltage winding of each core leg comprises the familiar stack of horizontal disc coils assembled over a conventional type cylindrical low voltage winding. It is common practice to provide shielding and grade the insulation of these disc coil stacks when designed for star connection and grounded neutral operation at the higher voltage levels. In general this shielding is secured by enveloping the high voltage stack with a number of line potential shields which are so shaped and disposed as to supply each individual disc coil with its required charging current.

According to the present invention, so far as it relates to the windings, the high voltage winding of each core leg comprises a group of vertical helical coils which are concentrically distributed between a line potential cylindrical shield and the low voltage winding. In general the high voltage coils are series connected between line and neutral terminals, and progressively spaced from the low voltage winding and grounded end surfaces in relation to the individual coil potentials from ground. With this distributed arrangement of the high voltage winding, adequate shielding is secured with a single cylindrical shield on each leg of the core, and the several coil to coil insulations not only serve individually as internal winding insulation, but also function in series as graded major insulation to the low voltage winding and ground. Positive axial separation of the conductors in the coils is provided, and individual coils may be readily clamped and handled in the shop or replaced in the field as independent units. It is also possible by this construction to obtain highly desirable electric and thermal characteristics including low inherent reactance and ready adaptability to forced cooling fluid flow directed through the coil ducts.

Further features of the invention relate to the insulation of the wire in the coils by means of wound in spacers, to the manner of construction of the electrostatic shield, to the connection of the coils, the circulation of the cooling fluid, to the spacing of the insulation and to other features, all hereinafter referred to more particularly in detail in the following specification and claims.

In the drawings:

Figure 1 is a sectional elevation taken through a single phase transformer constructed according to the present invention, having a group of vertical helical coils concentrically distributed between a line potential cylindrical shield and low voltage winding on each leg of the core,

Figure 2 is a plan view partly broken away,

Figure 3 is a section iOn the line 3--3 of Figure 1,

Fligure 4 is a section on the line 4-4 of Figure 2,

Figure 5 is a sectional elevation showing the arrangement of the coil about the core, the latter being shown diagrammatically,

Figure 6 is a plan view of the arrangement shown in Figure 5,

Figure 7 is a circuit diagram showing the circuit connections for the coil shown in Figures 1 and 5,

Figure 8 is a sectional elevation through the end of high voltage winding showing the woundin spacer construction and other details,

Figure 9 is a circu't diagram showing the arrangement with terminal connections attached for the high and low voltage coils,

Figure 10 is a sectional elevation, partially diagrammatic, showing means for effecting forced cooling fluid circulation through the transformer coils,

Figure 11 is a circuit diagram of suitable connection for the high voltage coils as applied to a single phase design,

Figure 12 is a circuit diagram of one leg of a three-phase design or one of two multiple legs of a single phase design,

Figure 13 shows a circuit diagram of an alternative form of connection for the single phase design,

Figure 14 shows an alternative arrangement ofa connection for the high voltage coils which may be used in any of the designs,

Figure 15 is a sectional elevation of one of the electrostatic shields extending around the high voltage coils,

Figure 16 is a plan View of the shield shown in Figure 15, certain parts being broken away and shown in section,

3 Figure 17 is a detailed sectional view of one end of the electrostatic shield drawn toan enlarged scale,

Figure 18 is a detail in plan showing a gap in the electrostatic shield,

Figure 19 is a detail in sectional elevation show-- ing the manner in which the electrostatic shield is built up,

Figure 20 is a detailed elevation, partly in perspective, showing the end insulation for the end support of the high voltage winding illustrating the manner of assembling same,

Figure 21 is a perspective view showing thespacing blocks for the end support, of the high voltage winding,

Figure 22 is a perspective view showing one of the elements constituting the spacing block,

Figure 23 is a perspective view of one of the aforesaid tie straps.

In the drawings like characters of reference indicate corresponding parts in all the figures.

Referring to the drawings, A indicates a suitable magnetic core, about the legs l-D of which are wound the conventional type helical low voltage coils B, which in turn are surroundedby a group of' vertical helical high voltage coils C concentrically distributed between the lowvoltage Winding and aline potential cylindrical shield D. The high voltage coils are formed from a group of fiat wires separated by suitable insulation and. spirally wound on hard foundation cylinders E of suitable insulating material. Each complete high voltage coil is separated from the rest by a series of axial spacers H uniformly distributed around the circumference ofeach high voltage coil; the space between the spacers serving for the axial circulation of oil or other The ends of the high voltage coils are provided with insulated mountings with radial oil passages as hereinafter described.

The magnetic core A may be of any conventional design, that illustrated having upright members at having any suitable, connection with the transverse members 2).

The low voltage winding B may be either the conventional type with horizontal ducts and intermittent spacers or beof the same general type as the high voltage Winding. In the latter case, which is more desirable when, usedin connection with directed forced flow of cooling fluid, the number of coils willbe substantially less than the number of high voltagecoils; The low voltage coil may be conveniently mounted on a hard foundation. cylinder ['2' of suitable insulating mater Each high voltage coil 0, asindicated-in Figure 8, is of helical structure having multiple insulated wireslforming rectangular conductors l3 each encased" in a continuous press-board spacer M of channel section. The spacer is formedfro m acoil of' a continuous channelsection which is simultaneously wound with the conductor onto the foundation cylinder F..- Ordinarily the spacer is a of an inch thick and the spacer flanges ext'end" radially at least A of an inch beyond the coil.

In this operation the conductor l3is inserted in the channel section and'both, are thenwound continuously and simultaneously aboutthe foundation cylinder E; both the conductorsandchannel sections being drawn from suitable supply coils.

The insulating end structure for each high voltage coil comprises a pressboardiendring l mounted on the end of the coil and annular collars vertical inder E l6 angular in cross section having a portion adjacent to the foundation cyland a horizontal portion extending outwardly across the top of the end of the high voltage coil adjacent to the end ring l5 and being designed to form part of the horizontal channel H which provides for the circulation of the oil. This insulating construction is the same on each high voltage coil and at each end thereof. In this way the horizontal portions of each of the collars I5 form the top and bottom of the horizontal radial passages l1 and are spaced and supported in position by spacers l8, the particular form of which is shown in Figures 20-23. By reference to these figures it will be seen that each spacer is formed from a series of rectangular sheets. I9, of'fibreboard or other suitable insulating material, having slots 20 therein adapted to receive hair pin tie straps 2| of fibreboard or other suitable material, which retain a stack of the sheets about oneormoreinsulating rings 22. I

The insulating structure at the end of the high voltage and low' voltage cells may be supplemented by a pluralityofadditionalringshaped end sheets; In addition end sheets 23 and spacing blocks 25 retained in position byhair pintie members 26 similar in form to the tie members 21- may be'providedat each end of the coil blocks B and C.

The construction of the shield D is shown in Figures1-5-1-7 where SKI-represents a hard (foundation cylinder whichsupports the shield structure proper, the foundation cylinder and shield being assembled" as a unitover the spacers H ofthe outerhighvoltage coila Over this-foundation cylinder is wound an insulating cylinder 5| of wound paper or fibr-eboard', say of an inch in thickness, theendof the cylinder 5 being extended, slotted and flanged over the ends of the shield as shown in Figure 17. About the cylinder 51 is wound the copper electrostatic shield 53. This electrostatic shield is conveniently-formedfrom a thincopperstrip 1 x .005 the adj acentturns ofwhich are maintainedseparate from each otherby'a paper envelope woundon the copper strip; as shownin-F'igure 19; from which it will'be-seen that a strip ofpaper 54 for example .01-"- thick, extends along the innerfaceof the copperstrip and'is then' folded backover the edges; The outer side of theelectrostatic shield 53-is covered-'byanother'oylinder of- Wound paperinsulatiom 55'; which may be about of aninch thick.

It-isnecessaryto-provide-a gap 56 in theelectrostaticshield" to prevent induced voltage being; built upin the circiunferentia'lly wound metal strip which; without a gap; Would-act as a transformercoil. 'I his gap is formed as shownin Figure 18:

The arrangementof the vertical and axial oil passages in the transformer already described permitsthe easy application of asystem of ex-- ternal' force flow heat exchangeby' theuse of the arrangement shownin- Figure- 10 in which thetransformer is enclosed in a casing 66 and a bafile; or barrier 67' isconstructed between the topand bottonr of the transformer, the upperpart being connected to a force pump -69 and heat exchanger- 68- whichdischarges intothe lowerpart' ofthecontainer; whereby the forced circulation of oil maybe-caused to take place-as indicated-- by the arrows.

The circuit" arrangements for the transformer are indicated in-Figs.- 7; 9, 1-1; 12, 13-and14'; In

Fig. '7, indicates the electrostatic shields under which the high voltage coils 1| are concentrically arranged, a plurality of variable taps being provided for certain of these coils as indicated in the drawings. It will also be observed that by this arrangement of coils it is possible to have horizontal cross-over sections 12 between adjacent coils. In this diagram, 13 indicates the high voltage neutral and M the high voltage line.

Figure 11 shows the circuit connection for a single phase design having two legs A and B as indicated in the drawings with the concentric high voltage coils extending between the shields and the low voltage winding as indicated on the diagram. Figure 12 shows the circuit arrangement for one leg of a three-phase design of conventional construction. Ordinarily from four to six coils are used for each leg corresponding to a test level range from 277 kv. to 576 kv. respectively. It will be seen that the winding provided in all figures except Figure 12 eliminates the objectionable and cumbersome vertical cross-over such as ordinarily used. The coil arrangement of Figure 13 is particularly applicable to a single phase design and is used in those cases where two multiple circuits are desirable in order to improve the space factor of the winding, taking into consideration the relation between the number of turns and current involved. The arrangement shown in Figure 14 is particularly applicable to a three-phase design and also has the advantage of eliminating vertical outside coil cross overs. In addition, this particular connection will be found to substantially improve the initial distribution of impulse voltage by reason of the shunt capacity of the physical coil cross overs.

In some cases where it is desired to improve the initial distribution of impulse voltage along the winding, the collar l6 may be made in the form of auxiliary shields, being constructed similarly to the electrostatic shield D and having the same cross-sectional form as the collar Hi.

It will be seen that the construction above described provides for positive axial separation of the conductors and that the individual coils may be readily clamped. and handled in the shop or replaced in the field as independent units. It will be found that the coil arrangement herein described will provide highly desirable electrical and thermal characteristics, including low inherent reactance and ready adaptability to forced oil flow directed through the ducts.

The low inherent reactance referred to results from the radial distribution of high voltage ampere current through the major insulation. This makes possible a reduction in the size of the core with corresponding reduction in other physical factors and weight.

It will be observed further that the outer surface of each high voltage coil is directly exposed to vertical oil ducts and the heat flow from the conductor is principally outward although some loss is transmitted through the foundation cylinders. Thus, for any horizontal section taken through a coil, the temperature rise of each strand above the local oil is readly determined by solution of an elementary series-multiple thermal circuit, once the thermal constants are establshed. The constants for solid insulation are well known, and a series of experimental direct current heat runs indicated that the continuous spacer flanges, extending into the axial duct, only increase the oil film constant of the vertical surface by about one degree for the usual design proportions. This surprisingly small effect of the spacer projections is apparently due to relatively high velocity eddy currents in the oil flow over the serrated coil surface. In general the relation between copper-oil gradient and load current is similar to that of conventional designs, and accepted rules definin permissible overloading of conventional type transformers should. be equally applicable to the distributed concentric design.

Unless some form of shielding is provided, it is well known that the initial distribution of impulse voltage along a transformer winding departs radically from uniformity, and subsequent voltage oscillations ordinarily raise the potential of intermediate winding points to values which preclude any grading of the insulation. It is clear, however; that shielding to establish a perfectly uniform initial distribution, with absolute elimination of subsequent oscillations, is neither attainable nor necessary in practical design. The degree of shielding to be used in a transformer with graded insulation is therefore a question of economic balance, and is determined by that combination of compromise shielding and partially graded insulation which has minimum overall cost.

With a vertical front impulse applied to the line terminal, the initial or electrostatic voltage distribution along the winding of the transformer is fixed by the combined network of radial shunt capacities and axial series capacities, including the shunt capacities associated with the physical coil cross-overs.

Because of the physical distribution of the high voltage winding through the major insulation, it is evident that both low frequency and impulse voltage stresses are advantageously distributed throughout the whole dielectric structure. Adequate control of impulse voltage on each leg of the core is readily secured with a single shield surface of plain cylindrical shape. Also greatly reduced and simplified shield insulation is sufiicient, since the maximum voltage from shield to adjacent winding is ordinarily less than one-third of line voltage, as compared to approximately full voltage in conventional construction.

As further compared to conventional disc coil construction, the structure of the presentinvention comprises a solid cylindrical column of copper and insulation in which the shearing and cantilever actions of conical shaped disc coil stacks are eliminated. Also the bearing surface between turns is continuous and uniform without the compression stress concentrations associated with intermittent radial spacers, and the entire cross section functions in the transmission of axial forces. It is further evident that the continuous spacer feature eliminates the possibility of misaligned spacer columns, and the hazard of cutting the conductor insulation at the spacer edges. Intermittent radial spacers are used in the supporting end insulation structure, but the continuous bearing feature within the coil is maintained at the coil ends by the beam strength of the end ring and adjacent flanged collar combined.

The continuous form of channel spacer M forms a very effective means of insulating the conductor and one which may be readily produced on an automatic folding machine. Its use enables a large number of intermittent radial spacers, which would have to be separately formed, to be done away with.

The various tap connections and terminal leads for the high voltage coils may be readily led out through the vertical and horizontal ducts.

The advantages resulting from the design herein 1 disclosed include the following:

(1') Reduced:inherentreactance, resulting in' a reduction of approximately "10 .per cent in total weight of critical materials required for a :given transformer specification.

(2) Ready adaptability to forcedoil 'fio'wdirected through the windings, "resulting in an increase of approximately 20 l per cent in thermal capacity of a given transformer and external cooler, or a reduction of approximately 15 per-cent in totalweight'of critical materials required for a given kv'anoutput.

('3) Strategic distribution of low'frequency'and impulse voltage stresses throughout the "whole dielectric structure, including'areduction of approximately 70 per cent in the voltage between shieldand windings, the advantageous distribution 'of all voltage stresses resulting from *the physical distribution of the high voltagewi'nding through the major insulation.

(4) Simple and'stable mechanicalconstruction of high voltagcwinding, insulation, and shield, including a new and improved form of helical coil structure and associated assembly which -are mechanically suitable for application in large high voltage power transformers.

(5) Theprovisi'on'of the spacers, both vertical and radial in the high-'voltage-coil substantially strengthens the structure formed by the combined coils.

Various modifications may be made'inthis-invention without departing fromth'e spirit thereof or the scope of the claimsyan'd, tli'erefore, the

exact forms shown are to be taken as illustrative only and not'in a-limiting sense, and'it is desired that only such limitations:shall'beplaced thereon as are disclosed in the prior artorare s'e't forth in the accompanying claims.

I claim:

1. In a power transformer having a- 'core and a low voltage winding e'xtendi'ng about tlie-core, a series of 1 spaced concentric helical high "voltage coils extending about "the low -'vo1tage=coil, a channel shaped insulating spacerin the-form of a continuous helix extending aboutjthe conductors of the high voltage coils and'maintaining positive axial separation thereof,- a horizontal end duct for cooling fluid at each-end of each'high voltage-coil,and a vertical duct between each two adjacent high 'volta'gecoils and communicating at each 'end'with anend duct, the'edges of said spacer 'extending radially'beyond said conductors to provideaccess of cooling fluid to a-surface of theconductor-salong their entire length.

2. In a fluid-cooled power'transformer having concentrically positioned helically coiled conducting-elementsin layers, spacing structure comprisinga pluralityof spaced, axially extendingcircularlyrpositioned insulating spacers between adjac'entlayers of conducting elements, and a chan nel shaped insulating spacer in the form of a continuous helix containing said conducting elements within its'channel, the edges of said last mentionedsp'acer extending beyond the conducting elements therein into abutment with said axially extending spacers, whereby clearance is provided-between said axially extending spacers and said -conducting elements.

MONTVILLE BURGESS M-AL-LET'I.

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